Origin of metabasites from upper tectonic unit of the Lavrion area (SE Attica, Greece): Geochemical implications for dual origin with distinct provenance of blueschist and greenschistʹs protoliths

The Lavrion area forms the westernmost part of the Attic–Cycladic crystalline belt (ACCB) and is built up by two tectonic units metamorphosed at HP/LT conditions. In the Upper Tectonic Unit metabasic rocks occur as greenschists and blueschists. Major and trace elements plotted against Mg# show a systematic increase in TiO2, Fe2O3*, Na2O, Zr, Y, V, La and Yb and a decrease in Al2O3, Ni and Cr with decreasing Mg#. Typically, the blueschists always exhibit a more evolved basaltic composition. The greenschists are characterized by LREE-depleted chondrite-normalized REE patterns and Zr/Nb values that range from 15 to 24. The blueschists are characterized by slightly LREE-enriched chondrite-normalized REE patterns and lower Zr/Nb ratios (6.6–11.3). Both rock types share common geochemical features like flat HFSE patterns or slight positive Nb anomalies with La/Nb < 1. The protoliths of both greenschists and blueschists show that two different suites generated their protoliths and that their magmatic evolution at low pressures has also been different. The observed Zr/Nb vs. Ce/Y ratios span the compositions of greenschists and blueschists out on hyperbolae at an ideal spectrum from intermediate N-MORB to E-MORB. However, the REE patterns of the studied metabasites cannot be explained by fractional crystallization processes alone. We interpret that the protoliths of the Lavrion metabasites support a dual origin; the blueschistʹs protoliths are comparable to rift-related mildly alkaline basalts whereas the greenschistʹs protoliths probably formed as typical N-MORB at an oceanic spreading center or in an evolved back-arc basin. A mélange setting with two distinct provenances of the basic protoliths is the proposed genetic model, similar to that envisioned for other parts of the ACCB, e.g. the islands of Syros, Sifnos and Tinos.